strip_colour: Strip '\x0F' (^O, formatting off)

Reported by @ssbr on freenode:

chmode `+c` doesn't strip `^O`, which turns off all previous formatting. This can cause clients that internally use mIRC formatting to render messages weirdly, e.g. [highlighted messages in HexChat](https://i.imgur.com/eDX8Aif.png).